Giving permissions to a node to multiple accounts without creating a new role

Question

I've built a site where all of the content is essentially private. All the relevant content types are Unpublished and my users (role=Owner) only have permissions to view/edit/delete their own Content. This is working just fine.

However, requirements have changed and I'm trying to figure out what to do. The new requirement is that an Owner can give permission to another user (role=OwnerAssistant) to view/edit/delete their data. I'll refer to the group of users that all administer the same dataset as an OwnerGroup.

It will not be feasible to create a Role for each of these groups because each new customer would need a separate role and we're hopefully going to have 50,000+ customers.

How can I control the permissions on a node so that a specific list of users can view/edit/delete that content? I'm ok with maintaining my own DB table that tracks the mapping of OwnerAssistants to Owners so that I have a way of determine if an OwnerAssistant is allowed to access content created by the Owner. However, I don't know how to let drupal know that uids 234, 894, and 1029 have x,y,z permissions to node 24590.

I will likely be integrating this with the Field Permissions module because I want to hide certain fields from OwnerAssistants.

Any help is appreciated!